Fix an unicode error whan installing current package in editable mode

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 - Fixed the `--no-dev` option in `install` not working properly.
 - Fixed prereleases being selected even if another constraint conflicted with them.
+- Fixed an error when installing current package in development mode if the generated `setup.py` had special characters.
 
 
 ## [0.12.2] - 2018-10-17

poetry/console/commands/install.py

@@ -75,7 +75,7 @@ exist it will look for <comment>pyproject.toml</> and do the same.
         else:
             builder = SdistBuilder(self.poetry, NullEnv(), NullIO())
 
-            with setup.open("w") as f:
+            with setup.open("w", encoding="utf-8") as f:
                 f.write(decode(builder.build_setup()))
 
         try: